/*
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance
 * with the License.  You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.apache.flink.api.java.sampling;

import org.apache.flink.annotation.Internal;
import org.apache.flink.util.Preconditions;
import org.apache.flink.util.XORShiftRandom;

import org.apache.commons.math3.distribution.PoissonDistribution;

import java.util.Iterator;
import java.util.Random;

/**
 * A sampler implementation based on the Poisson Distribution. While sampling elements with fraction
 * and replacement, the selected number of each element follows a given poisson distribution.
 *
 * @param <T> The type of sample.
 * @see <a href="https://en.wikipedia.org/wiki/Poisson_distribution">https://en.wikipedia.org/wiki/Poisson_distribution</a>
 * @see <a href="http://erikerlandson.github.io/blog/2014/09/11/faster-random-samples-with-gap-sampling/">Gap Sampling</a>
 */
@Internal
public class PoissonSampler<T> extends RandomSampler<T> {

	private PoissonDistribution poissonDistribution;
	private final double fraction;
	private final Random random;

	// THRESHOLD is a tuning parameter for choosing sampling method according to the fraction.
	private static final double THRESHOLD = 0.4;

	/**
	 * Create a poisson sampler which can sample elements with replacement.
	 *
	 * @param fraction The expected count of each element.
	 * @param seed     Random number generator seed for internal PoissonDistribution.
	 */
	public PoissonSampler(double fraction, long seed) {
		Preconditions.checkArgument(fraction >= 0, "fraction should be positive.");
		this.fraction = fraction;
		if (this.fraction > 0) {
			this.poissonDistribution = new PoissonDistribution(fraction);
			this.poissonDistribution.reseedRandomGenerator(seed);
		}
		this.random = new XORShiftRandom(seed);
	}

	/**
	 * Create a poisson sampler which can sample elements with replacement.
	 *
	 * @param fraction The expected count of each element.
	 */
	public PoissonSampler(double fraction) {
		Preconditions.checkArgument(fraction >= 0, "fraction should be non-negative.");
		this.fraction = fraction;
		if (this.fraction > 0) {
			this.poissonDistribution = new PoissonDistribution(fraction);
		}
		this.random = new XORShiftRandom();
	}

	/**
	 * Sample the input elements, for each input element, generate its count following a poisson
	 * distribution.
	 *
	 * @param input Elements to be sampled.
	 * @return The sampled result which is lazy computed upon input elements.
	 */
	@Override
	public Iterator<T> sample(final Iterator<T> input) {
		if (fraction == 0) {
			return emptyIterable;
		}

		return new SampledIterator<T>() {
			T currentElement;
			int currentCount = 0;

			@Override
			public boolean hasNext() {
				if (currentCount > 0) {
					return true;
				} else {
					samplingProcess();
					if (currentCount > 0) {
						return true;
					} else {
						return false;
					}
				}
			}

			@Override
			public T next() {
				if (currentCount <= 0) {
					samplingProcess();
				}
				currentCount--;
				return currentElement;
			}

			public int poisson_ge1(double p) {
				// sample 'k' from Poisson(p), conditioned to k >= 1.
				double q = Math.pow(Math.E, -p);
				// simulate a poisson trial such that k >= 1.
				double t = q + (1 - q) * random.nextDouble();
				int k = 1;
				// continue standard poisson generation trials.
				t = t * random.nextDouble();
				while (t > q) {
					k++;
					t = t * random.nextDouble();
				}
				return k;
			}

			private void skipGapElements(int num) {
				// skip the elements that occurrence number is zero.
				int elementCount = 0;
				while (input.hasNext() && elementCount < num) {
					currentElement = input.next();
					elementCount++;
				}
			}

			private void samplingProcess() {
				if (fraction <= THRESHOLD) {
					double u = Math.max(random.nextDouble(), EPSILON);
					int gap = (int) (Math.log(u) / -fraction);
					skipGapElements(gap);
					if (input.hasNext()) {
						currentElement = input.next();
						currentCount = poisson_ge1(fraction);
					}
				} else {
					while (input.hasNext()) {
						currentElement = input.next();
						currentCount = poissonDistribution.sample();
						if (currentCount > 0) {
							break;
						}
					}
				}
			}
		};
	}
}
